Thymic nurse cells (TNCs) are specialized epithelial components that contain T cells completely enclosed in intra-cytoplasmic vacuoles. Very little information about their function has been reported because of the lack of pure TNC populations in quantities necessary for their thorough analyses. However, they are believed to play a role in thymocyte development. Recently, our laboratory has been able to immortalize TNCs with SV40 virus. Fortunately, cells from these TNC lines have been shown to maintain thymocyte internalization capabilities in vitro. Further, we have shown that the only cells bound by and internalized into TNCs are CD4+ CD8+ thymocytes. These cell lines will be used to determine TNC function in the thymus. Two strategies will be employed: (1a) TNC- specific monoclonal antibodies (mAb) will be generated in an attempt to identify the receptor/ligand involved in binding and internalization. (b) These antibodies will be used to remove TNCs from the thymus in organ culture. A-lymphoid thymi treated with anti-TNC specific antibody will be recolonized with double positive thymocytes. The phenotype of the T cells recovered will be determined and compared to that of thymocytes isolated from identical thymi re-seeded with TNCs to pinpoint TNC function in thymocyte development. These antibodies will also be used to develop TNC-minus mice in vivo studies. (2) Antibody preparations shown to block binding of thymocytes to TNCs will be used to clone the gene(s) expressing the protein(s) involved. These studies are designed to determine the overall function of the thymocyte internalization by TNCs in the thymus.